Ground terminal design for high rate direct to earth optical communications

ABSTRACT

Challenges of direct-to-Earth (DTE) laser communications (lasercom) between spacecraft in low-Earth orbit (LEO) or medium-Earth orbit (MEO) and ground terminals can include short duration transmission windows, long time gaps between the transmission windows, deleterious effects of atmospheric turbulence, and the inability to operate in cloudy weather. Direct-link optical communications systems described herein can have data rates that are high enough to empty high-capacity on-board buffer(s) (e.g., having a capacity of at least about 1 Tb to hundreds of Tb) of a spacecraft in a single pass lasting only tens of seconds to a few minutes (e.g., 1-15 minutes), and overprovisioning the buffer capacity accounts for variations in the latency between links. One or more distributed networks of compact optical ground terminals, connected via terrestrial data networks, receive and demodulate WDM optical data transmissions from a plurality of orbiting spacecraft (e.g., satellites).

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This application claims priority, under 35 U.S.C. § 119(e), from U.S.Application No. 62/101,976, filed Jan. 9, 2015 and titled “GroundTerminal Design for High Rate Direct to Earth Optical Communications,”U.S. Application No. 62/101,975, filed Jan. 9, 2015 and titled “LinkArchitecture and Spacecraft Terminal for High Rate Direct to EarthOptical Communications,” and U.S. Application No. 62/101,955, filed Jan.9, 2015 and titled “Network of Extremely High Burst Rate OpticalDownlinks,” and each of the foregoing applications is hereinincorporated by reference in its entirety.

GOVERNMENT SUPPORT

This invention was made with government support under Contract No.FAS721-05-C-0002 awarded by the U.S. Air Force. The government hascertain rights in the invention.

RELATED APPLICATIONS

This application is related to U.S. application Ser. No. 14/991,394,filed on Jan. 8, 2016, and titled “Network of Extremely High Burst RateOptical Downlinks,” and U.S. application Ser. No. 14,991,377, filed onJan. 8, 2016, and titled “Link Architecture and Spacecraft Terminal forHigh Rate Direct to Earth Optical Communications,” the contents of eachof which is hereby incorporated by reference in its entirety.

BACKGROUND

Existing methods of delivering data from Earth-orbiting satellites to aground stations fall into two general categories: sending radiotransmissions to a fixed ground site via a relay satellite in, forexample, geosynchronous Earth orbit (GEO) or sending radio transmissionsdirectly to a ground site when the Earth-orbiting satellite passes overthe ground site. Because of the long distances involved, and the paucityof GEO satellites, the geosynchronous relay approach is restricted inboth availability and data rate. Likewise, because of the shortconnection time and restricted burst rates, the direct transmission isalso restricted in its capability. In addition, Federal CommunicationsCommission (FCC) and other regulations concerning radio-frequency (RE)spectrum allocation may constrain the available bandwidth and linkavailability for satellite transmissions. As a consequence, datatransfer to ground networks from Earth-orbiting satellites presents asignificant bottleneck in the data collection capabilities ofpresent-day Earth-orbiting satellite systems. This bottleneck is gettingworse now that satellite missions are generating more data than existingRE systems can handle.

SUMMARY

Embodiments of the present invention include an apparatus for receivingan optical signal transmitted from a spacecraft via a free-space opticalcommunications channel. In some examples, the apparatus comprises atleast one telescope, a single-mode waveguide in optical communicationwith the at least one telescope, an optical amplifier in opticalcommunication with the single-mode waveguide and a receiver in opticalcommunication with the optical amplifier. In operation, the telescopereceives the optical signal, which is modulated at a rate of at leastabout 40 Gbps, from the spacecraft via the free-space opticalcommunications channel. The single-mode waveguide guides the opticalsignal from the telescope to the optical amplifier, which amplifies theoptical signal. And the receiver demodulates the optical signal at arate of at least about 40 Gbps.

The telescope may define at least one aperture having a diameter ofabout 10 cm to about 100 cm. In some cases, the apparatus includes aplurality of apertures, each of which receives a corresponding portionof the optical signal.

Examples of the apparatus may receive wavelength-division multiplexed(WDM) optical signals from the spacecraft. In these examples, thereceiver may comprise a wavelength-division demultiplexer in opticalcommunication with the optical amplifier and a plurality of detectors inoptical communication with the wavelength-division demultiplexer. Inoperation, wavelength-division demultiplexer separates the WDM opticalsignal into a plurality of optical signals, and the detectors demodulatethe optical signals. The apparatus may also include a coherent source inoptical communication with the receiver for coherently demodulating theoptical signals.

The apparatus may also include an uplink transmitter to transmitidentification and validation information to the spacecraft. In someinstances, the uplink transmitter triggers transmission of the opticalsignal by the spacecraft when the spacecraft is at an elevation of atleast about 20 degrees above the horizon. The apparatus may also includea controller, operably coupled to the plurality of receivers and theuplink transmitter, that senses at least one error associated with theoptical signal. The uplink transmitter may trigger another transmission,by the spacecraft, of at least a portion of the optical signal inresponse to the error.

The apparatus may further comprise an adaptive optical element inoptical communication with the at least one telescope and thesingle-mode fiber. In operation, the adaptive optical element modulatesa spatial phase profile of the optical signal so as to compensate foratmospheric effects in the free-space optical communications channel.

Yet another embodiment of the present technology comprises a groundterminal for receiving a wavelength-division multiplexed (WDM) opticalsignal transmitted from a spacecraft. The ground terminal may comprise aplurality of telescopes in optical communication with the spacecraft, aplurality of optical amplifiers in optical communication with theplurality of telescopes, a coherent combiner in optical communicationwith the plurality of optical amplifiers, a wavelength-divisiondemultiplexer in optical communication with the coherent combiner, and aplurality of receivers in optical communication with thewavelength-division demultiplexer. In operation, the telescopes receivecorresponding portions of the WDM optical signal from the spacecraft,The optical amplifiers amplify the corresponding portions of the WDMoptical signal, which are coherently combined by the coherent combinerinto a coherently combined WDM optical signal. The wavelength-divisiondemultiplexer separates the coherently combined WDM optical signal intoa plurality of optical signals, which are demodulated by the receivers.

It should be appreciated that all combinations of the foregoing conceptsand additional concepts discussed in greater detail below (provided suchconcepts are not mutually inconsistent) are contemplated as being partof the inventive subject matter disclosed herein. In particular, allcombinations of claimed subject matter appearing at the end of thisdisclosure are contemplated as being part of the inventive subjectmatter disclosed herein. It should also be appreciated that terminologyexplicitly employed herein that also may appear in any disclosureincorporated by reference should be accorded a meaning most consistentwith the particular concepts disclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The skilled artisan will understand that the drawings primarily are forillustrative purposes and are not intended to limit the scope of theinventive subject matter described herein. The drawings are notnecessarily to scale; in some instances, various aspects of theinventive subject matter disclosed herein may be shown exaggerated orenlarged in the drawings to facilitate an understanding of differentfeatures. In the drawings, like reference characters generally refer tolike features (e.g., functionally similar and/or structurally similarelements).

FIG. 1A is a block diagram of a free-space optical communications systemthat supports transmissions of bursts at data rates of 10 Gbps or more.

FIG. 1B is a flow diagram that illustrates free-space opticalcommunications using the system shown in FIG. 1A.

FIG. 1C is a rendering of an implementation of the communications systemof FIG. 1A as part of a space-to-ground communication network.

FIG. 1D illustrates how the elevation angle changes as the spacecraftpasses over a remote terminal on Earth.

FIG. 2A is a block diagram showing components of space and groundsegments of a direct downlink communications system.

FIG. 2B is a flow diagram that illustrates free-space opticalcommunications using the system shown in FIG. 1A.

FIG. 3 is a schematic drawing of a receiver with a single aperture.

FIG. 4 is a schematic drawing of a receiver that includes severalapertures and that is configured to perform non-coherent aperturecombining and demodulation.

FIG. 5 is a schematic drawing of a receiver that includes severalapertures optically coupled to respective optical stretchers and that isconfigured to perform coherent aperture combining (before amplification)and demodulation.

FIG. 6 is a schematic drawing of a receiver that includes severalapertures optically coupled to respective optical amplifiers and opticalstretchers and that is configured to perform coherent aperture combining(after amplification) and demodulation.

FIG. 7 is a schematic drawing of a receiver that includes severalapertures and that is configured to perform channel-wise aperturecombining and coherent optical demodulation.

FIG. 8 is a schematic drawing of a receiver that includes severalapertures and that is configured to perform intermediate frequency (IF)coherent channel-wise aperture combining and coherent demodulationperformed electronically at the IF.

FIG. 9 is a schematic drawing of a receiver that includes severalapertures and that is configured to perform digitally coherentchannel-wise aperture combining and coherent demodulation.

FIG. 10A is a block diagram showing components of an adaptive opticsassembly suitable for use in a receiver at a remote terminal.

FIG. 10B is a rendering of the wavefront sensor of FIG. 10A sensing anunaberrated wavefront.

FIG. 10C is a rendering of the wavefront sensor of FIG. 10A sensing awavefront aberrated by atmospheric turbulence.

FIG. 10D is a rendering of the wavefront sensor and the deformablemirror of FIG. 10A acting on an aberrated wavefront.

FIG. 10E includes schematic drawings of the adaptive optics assembly ofFIG. 10A on a telescope in a receiver at a remote terminal.

FIG. 11A is a plot showing average coupling loss versus the ratio ofaperture diameter D to spatial coherence length r0 in atmosphere forlight entering a telescope and an optical fiber for a nominal atmosphere(Case 1) and a stressing atmosphere (Case 2).

FIG. 11B is a plot showing time variation of coupling losses for thenominal and stressing atmospheres of FIG. 11A.

FIG. 11C is a plot showing distribution functions for fade depths forthe nominal and stressing atmospheres of FIG. 11A.

DETAILED DESCRIPTION

The direct downlink communications systems described herein leverage theextremely short wavelengths of optical telecom signals to achieve freespace optical links that are only a few thousand kilometers long orless, and that deliver enough optical power (e.g., about 100 nW to a fewμW) to support extremely high data rates with compact, low-costsatellite terminals and compact, low-cost remote terminals on theground, in the air, or on other satellites. Such a system may include asatellite terminal that is small enough to be carried on amicrosatellite (e.g., a 1U-6U cubesat) in low-Earth orbit (LEO) and havea mass of about 3 kilograms or less. Burst rates supported by thesesatellite terminals can be nearly any rate supportable in the fibertelecom market, for example, several hundreds of gigabits per second upto multiple terabits per second. Thus, these satellite terminals havedirect downlink burst capabilities that can empty even very largestorage buffers (e.g., 1 Tb, 10 Tb, 100 Tb, 1000 Tb, or more) inminutes. Furthermore, although traditional optical satellitecommunication systems are hindered by atmospheric obstacles such asclouds, which can block laser beams and/or cause excessive transmissiondelays, the extremely high burst rates of systems described herein canbe used to transmit very large volumes of data under partly cloudyconditions, e.g., through the openings between clouds or otherobscurations, such as leaves or dust.

Challenges of direct-to-Earth (DIE) laser communications (lasercom) caninclude short duration contact windows (e.g., less than ten minutes)during which a successful transmission can occur, long time gaps (e.g.,tens of minutes to hours) between the transmission windows, limitedon-board data storage, deleterious effects of atmospheric turbulence,especially at low elevation angles, and the inability to operate incloudy weather. Direct-link optical communications systems describedherein can have data rates that are high enough to empty thehigh-capacity on-board buffer(s) (e.g., having a capacity of at leastabout 1 Tb to hundreds of Tb) of a satellite in a single pass lastingonly tens of seconds to a few minutes (e.g., 1-15 minutes).

In some embodiments, the median link latency does not exceed the bufferfill time for a given data acquisition rate. In other words, the buffercapacity and/or link latency may be selected so that the buffer is notbe completely filled by sensor data between links. Overprovisioning thebuffer capacity accounts for variations in the latency between links dueto weather, etc.

In some embodiments, one or more distributed networks of compact opticalground terminals, connected via terrestrial data networks, receive datatransmissions from a plurality of orbiting spacecraft (e.g.,satellites). When a ground terminal site is obscured by clouds, anoptical transmitter of the spacecraft sends buffered data to a nextopen/non-obscured ground terminal in the one or more distributednetworks of compact optical ground terminals. Compact, low-cost spaceterminals described herein can be proliferated so as to increase thetotal number of interactions between the constellation of spaceterminals and the terrestrial data networks. Alternatively or inaddition, inter-satellite crosslinks can be established within thecommunication system such that any single user (e.g., a satelliteseeking to transmit data) can access an open terminal (e.g., a spaceterminal in orbit) at any time.

Direct-Link Optical Communications System

Turning now to the drawings, FIG. 1A is a block diagram of a direct-linkoptical communications system 100. The communications system 100includes a spacecraft 110 (e.g., a satellite, such as a microsatellite,cubesat, etc.) in LEO or medium-Earth orbit (MEO) with one or moresensors 112 or other data-gathering devices that acquire datacontinuously, periodically, or aperiodically. One or more buffers 120 onthe satellite store the data from the sensors for transmission by anoptical transmitter 130 on the spacecraft 110 to a receiver 150 at aremote terminal 140 located on Earth. These transmissions may includeone or more short bursts 135 (e.g., 10-second to 10-minute bursts) ofdata modulated at extremely high data rate (e.g., at least about 10Gigabits per second (Gbps), at least about 40 Gbps, at least about 100Gbps, at least about 200 Gbps, at least about 1 Terabit per second(Tbps), at least about 5 Tbps, or at least about 10 Tbps, depending onthe application). The remote terminal 140, positioned at an altitude ofless than about 100,000 feet (e.g., on the ground; on a building orstructure; on a boat, buoy, or other ocean-based platform; or on anairplane, helicopter, unmanned aerial vehicle, balloon, or aircraftflying or hovering over the ground), receives the data at an averagerate of at least about 10 Terabits (Tbits) per day, or at least about100 Tbits per day. The remote terminal 140 includes one or more buffers180 that store the data received from the optical transmitter 130 fortransmission to one or more users via a communications network, such asthe Internet.

These data transfer rates and volumes enable the satellite to generateand store data at a daily average rate of about 300 Mbps or at leastabout 1,200 Mbps, if not more. The remote terminal 140 stores the datain the one or more buffers 180 at an average rate of at least about 300Mbps or at least about 1,200 Mbps and a burst rate of >10 Gbps. Theseaverage rates are computed over the span of a long enough period, suchas an hour, several hours, a day, several days, or a week and could behigher depending on the modulation rates of the free-space opticalcommunications signals, the number of ground stations and satellites,and the number of passes per day.

FIG. 1B is a flow diagram that illustrates a free-space opticalcommunications process 160 performed by the direct-link opticalcommunications system 100 shown in FIG. 1A. During orbit and/or flight,the sensors 112 on the spacecraft 110 gather and/or create data and savethe data in the onboard buffers 120 (box 161). For instance, the sensors112 may include one or more imaging sensors, such as visible, infrared,or hyperspectral cameras, or any other suitable sensor, that generateEarth image data at a relatively low average rate (e.g., <10 Gbps, <1Gbps, <100 Mbps, <10 Mbps, etc.). The sensors 112 may also include RFsensors like wide-band radars, synthetic aperture radars, etc. thatsense objects or terrain on Earth. The sensors 112 may also generatescientific and/or telemetry data, including data about the spacecraft110's flight path and electromagnetic events detected by the sensors112.

The buffers 120 store the data for burst transmission by the opticaltransmitter 130 (box 162). In some cases, the data is encoded at arelatively low data rate (e.g., <10 Gbps, <1 Gbps, <100 Mbps, <10 Mbps,etc.) with a forward error correction (EEC) code before being stored inthe buffers 120. In other cases, the data is encoded with a FEC at ahigh data rate upon being retrieved from the buffers 120 fortransmission by the optical receiver 130.

FEC-encoded data comprises one or more code words. When data istransmitted through an atmospheric channel that has power fluctuationsthat last longer than the transmission time for multiple code words(e.g., for a code word transmission duration on the order ofmicroseconds in duration and a power fluctuation that lastsmilliseconds), an interleaver can be used to temporally interleave thesymbols of many code words over a duration of about 1 second. With thisapproach, each code word sees a fairly uniform distribution of powerfluctuations (as opposed to entire code words being erased as mightoccur without the interleaver) and approximately error free performancecan be achieved without large power margins to overcome the effects ofthe power fluctuations.

In some cases, however, it may not be possible or practical to utilizethe FEC techniques described above in the lowest-layer codes. Forexample, a commercial transceiver may employ proprietary codes designedfor a fiber transmission, which does not typically experience the powerfluctuations seen in the free space channel. Or the additional latencyand/or memory that would be used for a ˜1-second for greater)interleaver may be a problem. In such cases, errors that are notcorrected by the lower-layer codes are compensated for at higher layers,for example, via erasure-correcting FEC codes (a form of FEC designed tospecifically correct for erasures) and/or repeat-request protocols thatrequest retransmission, via the uplink/beacon, of lost frames/segments.

In some cases, however, it may not be possible to utilize the FECtechniques described above in the lowest-layer codes. For example, acommercial transceiver may employ proprietary codes designed for a fibertransmission, which does not typically experience the power fluctuationsseen in the free space channel. The receiver 150 initiates eachtransmission based on the spacecraft's trajectory, on-board data storagecapacity, previous transmissions from the spacecraft 110, and projectedtiming of future transmission windows (passes) from the spacecraft 110.Based on this information, the receiver 150 communicates with thespacecraft 110 via a low-bandwidth (e.g., 10 kbps) RF or optical uplink155 (box 163). The receiver 150 and the optical transmitter 130 alignthemselves with respect to each other, possibly using gimbals at theremote terminal 140 and the spacecraft 110 and/or body pointing by thespacecraft 110 (box 164). Alignment (or re-alignment) may occurcontinuously or as desired throughout the transmission process 160.Likewise, the spacecraft 110 and remote terminal 150 may communicatewith each other via the uplink throughout the transmission process 160.

Once the receiver 150 and the optical transmitter 130 are properlyaligned, the receiver 150 sends a control signal to the optical receiver130 via the uplink 155 to initiate data transmission by the satellite110 (box 165). In some cases, the receiver 150 detects or calculateswhen the spacecraft 110 reaches a predetermined angle above the horizon(for example, an angle below which operations are inefficient or belowwhich the data volume that can be transferred during the pass is lessthan desired) as described below with respect to FIGS. 11A-11C andstarts the transmission accordingly. The receiver 150 may also time thetransmission(s) to avoid clouds, atmospheric turbulence, etc. in othercases, the optical transmitter 130 may initiate transmission at apredetermined time (i.e., without an explicit signal from the groundterminal).

In response to receiving the control signal, the optical transmitter 130transfers, at as high a rate as is possible for the particular spaceterminal/remote terminal pair and link conditions, as much of the dataas possible in one or more free-space optical signal bursts 135 (box166). A burst 135 may comprise at least about 1 Terabyte of information,be modulated at a rate of Gbps (e.g., 40 Gbps, 100 Gbps, 1 Tbps, 10Tbps, etc.), and/or last up to several minutes. As explained in greaterdetail below, the free-space optical signal burst 135 can include aplurality of wavelength-division multiplexed (WDM) signals. Inoperation, the receiver 150 de-multiplexes the WDM signals in thefree-space optical signal burst 135.

The short link delay of the downlink allows for several options for dataflow and control, such as interleaving/forward error correction (FEC),simple repeats or erasure-correcting ITC, automatic repeat requests(ARQs), and/or delay/disruption tolerant networking (DTN). To controldata flow, the receiver 150 checks the quality of the bursts 135 that itreceives from the satellite, possibly using an FEC code (box 167),cyclic redundancy check (CRC), or other methods. Clouds, atmosphericfading, temperature gradients, misalignment, and other phenomena maydegrade the signal quality, e.g., by attenuating or distorting a givenburst 135. Severe enough attenuation or distortion may introduce biterrors at a frequency above the frequency at which the FEC can correcterrors.

Several mechanisms exist for detecting bit errors in transmitted,FEC-encoded data. At lower layers of the protocol stack (e.g., thephysical layer and/or the data link layer), errors in the received datamay be detected and/or corrected by an FEC decoder. A cyclic redundancycheck (CRC) code may also be appended to the data to facilitatedetection of errors that the FEC code does not detect. In someimplementations, frames/segments of data with uncorrectable errors arenot delivered to higher layers, and instead are “erased.” The higherlayers may detect such omissions and use additional protocols forcorrection, such as erasure-correcting FEC codes and/or repeat-requestprotocols that request retransmission, via the uplink/beacon, of lostframes/segments. Erased frames/segments can be detected using a sequencecounter that counts frames/segments and that increments with eachtransmitted frame/segment.

If the receiver 150 (or a processor coupled to the receiver 150) detectserrors based on the received FEC (box 168), it may re-align itself withrespect to the optical transmitter 130, boost its receiver gain (e.g.,by increasing local oscillator power for coherent detection), and/orsend a control signal to the satellite 110 via the uplink 155. The checkfor good data may occur on the time scale of a frame of data (e.g.,microseconds), and individual frames of data may be retransmitted iferrors are caused by, say, turbulence fluctuations.

The satellite 110 may respond to this control signal by re-transmittingsome or all of the degraded data (box 169) to the receiver 150. Beforere-transmitting the degraded data, the optical receiver 130 may re-alignitself with respect to the receiver 150 and/or boost its signal power inorder to increase the signal-to-noise ratio (SNR) at the receiver 150.It may also reduce the data transmission rate in response to the controlsignal.

In some embodiments (e.g., those with strong FEC), the check for gooddata may occur on a time scale of a link session. If a link session isdetermined to be had (e.g., due to poor atmospheric conditions), theentire session may be repeated at the next link. Put differently, if theprocessor at the receiver 150 or remote terminal 140 determines that thereceived data is corrupt or degraded after the satellite 110 has passedfrom view, it may signal, to another remote terminal via the groundnetwork, that the satellite should repeat the entire transmission duringthe link to the other remote terminal.

Data transmission proceeds when the optical receiver 150 hassuccessfully received the last frames (or other structure) of data fromthe optical transmitter 130. The optical receiver 150 may send anacknowledgment signal to the optical transmitter 130 in response toreceiving all of the data successfully. And if the optical transmitter130 determines that it will not be able to retransmit some or all of thedegraded data while the satellite 110 remains in view of the remoteterminal 140, it may instead continue to store the data in the buffer120 for transmission at the next opportunity.

Even when the link connection duration is short, the extremely highburst rate facilitates the download of huge amounts of stored data.Furthermore, although clouds are often considered to be the Achillesheel of laser communication, the spacecraft 110, when orbiting in LEO,can traverse the sky in a few tens of minutes, and under partly cloudyconditions can quickly link up with the remote terminal 140 via spacesbetween the clouds, and burst at extremely high data rates very largeamounts of stored data. For example, under clear (cloud-free) conditionsand at a transmission data rate of 200 Gbps, a 10-minute pass of thespacecraft 110 can be sufficient for downloading up to 15 Terabytes ofdata, depending upon the size of the buffer(s) 120. By extension, thecommunications system 100 can accomplish the transfer of up to 1.5Terabytes of data in a sky with only 10% clearings between the clouds.

The remote terminal 140 can store the received data from one or morepasses of the spacecraft 110 (e.g., via the data bursts received duringeach of the one or more passes), and can forward it to a user eitherimmediately or upon request. For example, a user may request aparticular set of data, and the system 100 may retrieve it from thecorresponding satellite via the next available remote terminal. Theremote terminal then forwards the received data to the user via aterrestrial data network, such as the Internet. In applications wherethe terrestrial data network is not widely distributed, this systemcould be used to distribute content to local caches around the globe.These caches may be connected to the Internet and/or to local users vialocal-area networks (e.g. WiFi, Cellular, etc.).

Satellite-Based Free-Space Optical Communications Networks

Because the satellite is in LEO or MEO, and the link is relativelyshort, the optical transmitter 130 and the receiver 150 can berelatively small, light, and inexpensive. And because the receiver 150can also be small, light, and/or inexpensive, the system 100 may includemany receivers 150 distributed over a wide area (e.g., the continentalUnited States) to increase the probability that the spacecraft 110 willbe in view of one receiver 150 before the spacecraft's buffer fills upand either stops recording data or starts overwriting data. Thereceivers 150 may be located at fixed sites, mobile sites, aircraft,and/or even other spacecraft equipped with optical transmitters 130 forrelaying data to Earth. In other words, communications systems 100 mayinclude a network of space-based optical transmitters 130 andground-based, airborne, and/or space-based optical receivers that can toestablish space-to-ground (LEO-to-ground), space-to-air, orspace-to-space (e.g., LEO-to-LEO or LEO-to-GEO) communications links.

The set-up and breakdown of these links can be coordinated withlow-bandwidth optical and/or RF uplinks according to a predetermineddata delivery protocol. In some instances, coordination and control ofthe space system (e.g., including the spacecraft 110 and, optionally,one or more further spacecraft) is performed using optical uplinks fromone or more remote terminals, and/or via RF communications with thespacecraft 110 and the optical transmitter 130. For instance, controlinformation (e.g., scheduling information, updated terminal location,software updates, etc.) may be delivered to the spacecraft 110 wheneverthe spacecraft 110 is in contact with a remote terminal 140, an RFterminal on the ground, or a space relay. Coordination and control ofthe ground system (e.g., including the one or more ground terminals 140and, optionally, a network interconnecting the ground terminals 140) canbe performed using ground connections and/or optically from the spacenetwork (e.g., for geographically isolated remote terminals 140). Inother embodiments, the optical uplink is omitted.

FIG. 1C shows how the communications system 100 of FIG. 1A can be usedto relay data from another satellite 170. The other satellite 170 andthe LEO satellite 110 exchange data via relatively low-bandwidth (e.g.,1-40 Gbps) one-way or two-way communications link 175, possibly overminutes or hours. The LEO satellite 110 stores the data from the othersatellite 170 in its on-board buffers 120, When the LEO satellite 110 isin view of a remote terminal 140 on Earth, it transmits the data to theremote terminal in a free-space optical signal burst 135 as describedabove and below.

System Components

FIG. 2A is a block diagram showing components of the high-data-ratedownlink optical transmitter 130 and the optical receiver 150 of thedirect downlink communications system 100. The direct downlinkcommunications system 100 may connected to a ground network at thereceiver 150 (e.g., via a buffer 180 on the receiver side).

An Example Optical Transmitter

The optical transmitter 130 can include one or more high-capacitybuffers 120 and one or more modems 230. The high-capacity buffers 120are configured to buffer data received at a first rate from one or morelocal data sources 211, including the sensors 112 shown in FIG. 1A, andare electrically coupled to the modems 230 to transfer the data to themodems 230 at a second rate that is higher than the first rate. The datamay be pre-processed via a data processor 212 prior to receipt at thebuffer(s) 120, for example, to insert error correction, requestresending of erroneous bits, and/or to exert feed forward control bydetecting and accounting for data errors. The data in the data sources211 can include scientific, metrology, position, and/or other types ofdata, that is collected during a spacecraft mission and stored betweenreadout sessions.

The modem(s) 230 can include power conditioning electronics (a powerconditioning “subsystem”), a digital data formatter and encoder, and ahigh-speed modulator configured to perform high-speed amplitude and/orphase modulation (e.g., quadrature phase shift keying (QPSK), quadratureamplitude modulation (QAM), etc.), and one or more master lasertransmitters 231 that emit light in the telecom C band (1530-1565 nm),for example, at 1550 nm. The outputs of the master laser transmitters231 may be spaced in the spectral domain at integer multiples of about50 GHz within this band. The modem(s) 230 receives buffered data fromthe buffer(s) 120, either via a serial channel or via parallel channels,and converts the buffered data into a plurality of modulated opticalsignals. In some implementations, the output speed of the buffer(s) 120is matched to the modulator.

To achieve the highest possible data rates, the space terminal canfurther include a fiber or Arrayed-Waveguide-Grating (AWG) wavelengthdivision multiplexer (WDM) 213 that is fed by multiple master lasertransmitters 231 of the modem(s) 230, operating at differentwavelengths. Other devices suitable for multiplexing the signals fromthe master laser transmitters 231 include, but are not limited tofused-taper fiber couplers, free-space dichroics, and other devicesknown in the art of fiber-optic communications. Optical signals receivedat the WDM 213 from the laser transmitters 231 are multiplexed by theWDM 213 into a single, multiplexed optical signal. The WDM 213 isoptically coupled (e.g., via an optical fiber, such as a single-modeoptical fiber) to an optical amplifier 215 (e.g., a semiconductoroptical amplifier or fiber amplifier, such as a C-band fiber amplifier)that amplifies the multiplexed optical signal (e.g., to a level of atleast about 100 mW to several watts, or at least about 500 mW to severalwatts) before it passes through an optical head 214. In someembodiments, the laser transmitter 231 is housed separately from themodem(s) 230 within the optical transmitter 130 of the communicationssystem 100.

The optical head 214 can comprise an optics assembly and, optionally, agimbal (e.g., a two-axis gimbal). The optics assembly of the opticalhead 214 can include one or more telescopes, including a downlinktelescope and an uplink telescope, each having an aperture with adiameter of between about 1 cm and about 5 cm. (In some cases, thedownlink and uplink may share an aperture, e.g., if a gimbal is used topoint the aperture.) The telescope can be fiber-coupled to the downlinkoptical transmitter 130 via a fiber-optic connection to the output ofamplifier 215 and configured to transmit a downlink beam/signal 235toward an optical receiver 150. Some optical terminals described hereinare configured to support lasercom link data rates of several hundredGbps or higher, with a total mass of less than about 5 kg and a totalpower consumption of about 50 W or less. Depending upon the embodiment,the data rate can be about 10 Gbps or more, about 40 Gbps or more, 100Gbps, 200 Gbps, 1 Tbps, 5 Tbps, or up to 10 Tbps.

The optical head 214 is also operably coupled to a relatively low datarate uplink receiver 216 (or “receiving detector”) having a wideacquisition field-of-view (e.g., 1 milliradian to about 50 milliradians)and configured to receive an uplink beacon from an optical receiver(e.g., of a remote terminal). The uplink receiver 216 may be operablycoupled to the downlink telescope of the optical head 214, or to afurther telescope within the optical head that is dedicated to thereceiver 216 and is co-aligned with the downlink telescope. The uplinkreceiver 216 has a field of view that is large enough to detect anuplink signal from the receiver 150 when the spacecraft 100 (and,optionally, a dedicated gimbal of the spacecraft) has pointed the opticsof the optical transmitter 130 toward the uplink source. (Note thatthere could also be a separate uplink data receiver, in addition to theacquisition receiver.)

When the uplink receiver 216 detects the uplink, it waits for amodulation (e.g., pulsed) which carries a unique identifier for theground station. In some embodiments, the uplink signal carries anencrypted message containing an identifier of the optical receiver 150,If the optical transmitter 130 determines (e.g., based on contents ofthe uplink signal) that the detected uplink is an expected one, theoptical transmitter 130's pointing can be fine-tuned so that the opticalhead 214 is pointed toward the optical receiver 150, at which time theoptical transmitter 130 sends the downlink beam/signal. The uplinkreceiver 216 continues to monitor the uplink signal for pointingcorrections and/or for link and data-flow control messages. Suchmessages could support, for instance, control of the optical transmitter130 pointing via motions of the downlink beam that the optical receiver150 detects as power variations.

There are a couple of specific cases of closed-loop point-aheadcorrection that could be considered. In one case, the spacecraft variesits pointing in a predetermined way and the receiver sends backinformation to correct a pointing bias based on its observations of theresulting power fluctuations. In another case, the receiver may commandthe spacecraft to adjust its pointing slightly in a particulardirection. Then, based on its measurement of the impact of that motionon the measured received power, the receiver could command furtheradjustments.

The optical receiver 216 is operably coupled to a controller 217(“control module” or “control electronics,” for example, including oneor more microprocessors). The optical receiver 216 sends uplink datareceived from the optical receiver 150 via the optical head 214 to thecontroller 217. The controller 217 is configured to control spacecraftand/or telescope pointing, connections to telemetry, and/or downlinkdata flow, and can be configured to monitor the “health” of opticalcomponents of the optical transmitter 130, the modem(s) 230, etc. Forexample, the modems 230, etc., may provide low-rate interfaces formonitoring their temperature, indications of faults in the receipt ortransmission of data, etc.

The controller 217 can have command and/or telemetry connections with aspacecraft bus. The controller 217 can include a memory that storespositions of existing terminals (e.g., other space terminals and/orremote/ground terminals), its own position and attitude (e.g., overtime), and/or a clock for synchronizing operations with the groundsegment 240. The controller 217 can control the acquisition and uplinkcommunication detector (i.e., optical receiver 216) and demodulate,validate, interpret, and/or act upon the uplinks. The controller 217 mayalso oversee the starting and stopping of the downlink data flow basedon clocks, terminal angles, and/or requests from the optical receiver150.

Steering of the optical transmitter 130 is performed by the two-axisgimbal optionally included within the optical head 214, and/or throughbody steering of the spacecraft itself, for example, if the spacecraftis a microsatellite, or with a small, fast-steering mirror. Thespacecraft and/or the optical transmitter 130 can include one or moreattitude sensors configured to measure the attitude of the opticaltransmitter 130.

In some embodiments, the optical transmitter is configured to opticallycrosslink high-speed data to other spacecraft (e.g., to other satelliteswithin a constellation of satellites). Such optical transmitters caninclude any or all of the components described above with regard to theoptical transmitter 130 of FIG. 2A, but may include larger telescopesand/or larger power amplifiers. Additionally, such optical transmittersmay send buffered data over crosslinks at a lower data rate than on adownlink because of larger diffraction losses and smaller receivetelescopes on the spacecraft with which the crosslink is established.However, cross-linkable optical transmitters may not require as full aset of data-handling protocols because of the all-vacuum nature ofcross-links (e.g., there is no atmospheric fading due to turbulence orclouds).

In some embodiments, the optical transmitter 130 of the communicationssystem 200 includes the optical head 214, the uplink acquisition andlow-data-rate optical receiver 216, a high-data-rate optical transmitterwith fast readout, and a control and spacecraft-interface function 217.

In some embodiments, the optical transmitter 130 includes one or moreopto-mechanical components, such as an opto-mechanical scanner. Since aLEO link can be established at a very high data rate with a relativelysmall spacecraft aperture (e.g., a few centimeters or less), theopto-mechanical systems for the space terminal can be much simpler thanthose developed for larger apertures (>10 cm). While one could simplyscale a more complex design to a smaller aperture, doing so would beunnecessarily expensive.

The performance of the optical link between the optical transmitter 130and the optical receiver 150 (“link performance”) can vary due to: (1)range variations as the spacecraft passes over the optical receiver 150;and/or (2) fading at the optical receiver 150 due to atmosphericturbulence, space terminal motion, clouds, etc., resulting in powerfluctuations at the optical receiver 150. The optical receiver 150 candetect these power changes, for example by monitoring power and/or errorperformance, and can send corrections or repeat-requests to the opticaltransmitter 130 via, the uplink using relatively low-rate signaling.

An Example Receiver

As shown in FIG. 2A, the receiver 150 can include an optical head 248communicatively coupled to a pointing, acquisition and tracking (PAT)module and/or an adaptive optics (AO) module 241 running one or more AOalgorithms. The optical head 248 includes a compact telescope (ormultiple telescopes, for example in an array) with a downlink aperturediameter of about 10 cm to about 100 cm (e.g., 20 cm, 30 cm, 40 cm, 50cm, 60 cm, etc.) and a demultiplexer. (larger telescope diameters, e.g.,for even higher-capacity links, are also contemplated.) The telescopecan be mounted on a two-axis gimbal for pointing anywhere in the sky andfor deployment almost anywhere on Earth. The compact design of thegimbal-mounted telescope allows for the telescope to be stationed onrooftops, car roofs, etc. In some embodiments, the telescope is mobile.Downlink signals received by the telescope of the optical head 248 aredemultiplexed into a plurality of optical signals that are passed to anoptical receiver 250, including one or more front ends 245 to convertthe optical signals into digital signals for further processing at oneor more digital processors 246 and, optionally, digital combining at 247as shown in FIG. 9. The processed digital signals are then passed to oneor more buffers 180 for storage and/or for communication to a user via aground network.

The optical head 248 is optically coupled to an uplink (UL) modem 243which transmits uplink signals to be sent to one or more space terminals210. Low-power (e.g., about 0.1 W to about several Watts) uplinktransmissions can be sent from the optical receiver 150 via a downlinkaperture of the telescope of the optical head 248 (i.e., a “shared”aperture), or via a small, dedicated, uplink-only telescope/aperture.The uplink optical power and aperture may be selected such that it isbelow levels of concern for local eye safety and/or the Federal AviationAdministration (FAA). The optical receiver 150 may be configured to sendan uplink transmission toward a selected/predetermined opticaltransmitter 130 at a selected/predetermined time so as to alert theoptical transmitter 130 that a link is desired.

The uplink transmission beam may be sufficiently wide to remove, reduceor mitigate as many pointing uncertainties as possible. Alternatively,the uplink transmission beam may be a narrow beam that is scanned acrossthe uncertainty region.

The uplink is modulated by the UL modem 243, and can carryidentification and/or validation signals, as discussed above. Shortly(e.g., seconds) after transmission of the uplink from the opticalreceiver 150, the downlink telescope of the optical head 248 may detecta returned beam, spatially acquire and lock up with the returned beam,and subsequently acquire, demodulate, decode, and otherwise process thedownlink data via the receiver 250. The processed data is stored in oneor more local buffers 180.

The optical receiver 150 also includes a controller 242 (“controlsmodule” or “control electronics,” for example, including one or moremicroprocessors) to control uplink telescope and/or receiver pointing,connections to telemetry, uplink data flow and/or downlink data flow.The controller 242 of the optical receiver 150 can be configured to: (1)oversee the AO algorithm; (2) calculate and implement the pointing ofthe gimbal based on knowledge or an estimate (e.g., position, orbit,trajectory, velocity, etc.) of the optical transmitter 130; (3)calculate and create data transmission protocol signals; (4) coordinateactivities of the integrated optical receiver 150; and/or (5)communicate with users and the ground data infrastructure.

A ground terminal 240 can include a GPS receiver or other means fordetermining its location, and may also include a star field tracker fordetermining its attitude. The optical receiver 150 can include a memorythat stores information about the satellites it can communicate with,along with their present ephemeris and orbital parameters.

A ground terminal 240 can include a mechanical, electro-optic, orelectronic turbulence mitigation system, which may use a small amount ofthe downlink power for its operation. The amount of the downlink powerused by the turbulence mitigation system can depend upon the brightnessof the received transmission from the space terminal and/or the durationof the link formed between the ground terminal 240 and the spaceterminal. Optical components of the optical receiver 150 can alsoinclude a weather protection subsystem, for example comprising one ormore apertures that are opened and closed depending upon weathermeasurements from dedicated monitors.

As mentioned above, the receiver 150 may be disposed at a groundterminal, on a boat, on a spacecraft, or on an airplane. Space-bornereceivers are positioned farthest from atmospheric turbulence, and socoupling from even a large space telescope into a fiber can berelatively straightforward.

However, far-field scintillation on the downlink can cause dropouts, andso multiple receive apertures, spaced apart from one another, can beused in space, in order to combat scintillation. That is, turbulence inthe atmosphere causes the power in the downlink beam to have somespatial distribution. A single small aperture might be located in a(temporary) null of the far field power distribution and, thus,experience a fade. With multiple spatially separated small apertures, itbecomes less likely that all apertures will be simultaneously located innulls in the far field power distribution. So the total power collectedby multiple apertures tends to fluctuate less than the power collectedby one small aperture. Note that “small” in this discussion refers tothe aperture size relative to the spatial coherence length of theatmosphere, which is typically about 1-20 cm.

Similar fading mitigation tradeoffs also exist when comparing systemsthat employ feed-forward, feedback, and modified optics designs. Groundterminals and other remote terminals can also include one or morereceive apertures, depending upon the design.

Receiver/Remote Terminal Operation

FIG. 2B shows a free-space optical communications process 280 performedby the receiver side of the direct-link optical communications system100 shown in FIG. 2A. During orbit of a spacecraft bearing an opticaltransmitter 130, a communications channel/link is established betweenthe optical transmitter 130 and a remote terminal bearing a receiver150. The optical transmitter 130 transmits a WDM optical signal via thecommunications link. The WDM optical signal is modulated, for example ata rate of at least about 40 Gbps, and is received at the receiver 150(box 282) via an aperture at the optical head 248 having a diameter, forexample, of about 20 cm to about 50 cm. Receiving the WDM optical signalcan include coupling light from the optical head 248 to awavelength-division demultiplexer via a single-mode fiber. Alternativelyor in addition, receiving the WDM optical signal can include modulatinga spatial phase profile of the WDM optical signal to compensate foratmospheric effects in the free-space optical communications channel.

Free-space optical communication described herein can be initiated bythe remote/ground terminal first illuminating the space terminal with anuplink “beacon.” In some implementations, the ground terminal transmitsinformation, such as identification and validation information (box281), via the beacon using the uplink modem 243. The ground terminalpoints the beacon at the space terminal, for example, based onpredictions of the spacecraft location obtained through spacecrafttracking and/or updated with orbit corrections based on previous linkacquisitions. The ground terminal beacon beam may be broad enough tocover its pointing uncertainty (arising, for example, due to groundterminal uncertainties and/or uncertainties in the spacecraft location),or a narrower beam may be scanned over the uncertainty region.

Data transmitted on the beacon may be used by the spacecraft forID/authentication of the corresponding ground terminal. The spacecraftmay validate this information and, if validation is successful, transmitthe WDM optical signal to the ground terminal in response. Inparticular, the space terminal can point its acquisition sensor, such asa quadrant detector or camera, in the general direction of the groundterminal. The field of view of the acquisition sensor is larger than thespacecraft pointing uncertainty (the sensor field-of-view (FOV) istypically about 1 mad to several degrees).

When the ground terminal illuminates the space terminal, the spaceterminal registers the signal on the acquisition sensor and slews itspointing to correct for any errors, thereby pointing its narrow downlinkbeam back to the ground terminal while accounting for the relativemotion of the terminals. For example, the correct pointing for thedownlink may be different than the apparent direction from which thedownlink signal is received (this is sometimes called “point ahead” or“look behind”). Once the space terminal is illuminating the groundterminal, the ground terminal begins tracking on the space terminalsignal in order to couple the received downlink light transmitted fromthe space terminal into the ground terminal receive fiber. Duringdownlink transmission, the uplink/beacon from the ground terminal may beused for supplemental purposes, such as to ensure reliable data deliveryby sending a repeat request to the space terminal upon detection of codeword errors in the downlink transmission.

Once the link has been established, the optical transmitter 130transmits a WDM optical signal via the communications link. The receiver150 separates the received WDM optical signal into a plurality ofoptical signals (box 283) and demodulates (e.g., coherently) at leastone optical signal of the plurality of optical signals (box 284). Insome cases, the receiver 150 is also configured to monitor transmissionerrors. For example, the receiver 150 detects or senses a bit error rate(BER) or other fidelity metrics of one or more signals of the pluralityof optical signals (box 285) and determines whether the BER exceeds apredetermined threshold (box 286), if the BER exceeds a predeterminedthreshold, the receiver 150 triggers another transmission (box 287) fromthe spacecraft, of at least a portion of the WDM optical signal.

In other configurations, free-space optical communication is notinitiated by the remote/ground terminal first illuminating the spaceterminal with a “beacon.” For example, if the space terminal beam isbroad enough that it could illuminate the ground terminal withsubstantial or completely certainty, given all of the various pointinguncertainties (s/c position and attitude knowledge, ground terminalposition), the ground terminal uplink/beacon may be unnecessary. Notethat since, as noted above, the uplink/beacon from the ground terminalmay be used for purposes such as sending repeat requests, those taskswould have to be handled differently if there is no uplink/beacon (e.g.with stronger forward error correction codes that reduce the incidenceof code word errors).

In most instances, operations are scheduled to occur based on groundterminal and space terminal locations so that the links occur when thegeometry (e.g., elevation, link distance, etc.) permits. In other words,the acquisition process does not start until the space terminal reachesa specified elevation angle based on predictions of its location, andacquisition stops when the space terminal falls below the specifiedelevation angle.

Receive-Side Architectures

FIGS. 3-9 show example optical receiver architectures for the receiver150 shown in FIGS. 1A and 2, As mentioned above, the receiver 150 may bedisposed at a ground terminal, on a spacecraft, or on an airplane.Space-borne receivers are positioned farthest from atmosphericturbulence, and so coupling from even a large space telescope into afiber can be relatively straightforward.

Single-Aperture Optical Receiver

FIG. 3 is a schematic drawing of a receive-side architecture of a remoteterminal including a single aperture defined by a telescope 350, forcoherent or non-coherent demodulation, according to an embodiment. Areceived downlink signal (e.g., comprising a plurality of sub-channels)passes through a telescope 360, which couples the downlink signal intoan adaptive optics (AO) sensor and tracking filter (e.g., a Kalmanfilter) 352 that applies one or more AO algorithms to the receivedsignal before coupling it to a fiber. AO algorithms correct the opticaleffects of atmospheric turbulence, thereby enhancing the performance ofthe remote terminal. An optical amplifier 353 receives the fiber-bound,AO-corrected signal and pre-amplifies it before passing the signal to ademultiplexer (De-WDM) 354 that converts the single multiplexed incomingsignal into a plurality of de-multiplexed signals (e.g., differentlycolored signals) that are fed to respective optical receivers 350A-350N.A local oscillator (LO) bank 355 is operatively coupled to the opticalreceivers 350A-350N and is configured to perform coherent (e.g.,homodyne, heterodyne, or intradyne) detection down of the coloredde-multiplexed signals received at the optical receivers 350A-350N. Thereceiver shown in FIG. 3 can also be used for non-coherent direction,such as direct detection or differential phase shift keying (DPSK). Insome embodiments, feed-forward or feedback techniques may be added tothe receive-side architecture of FIG. 3, for example to compensate forresidual fading effects (e.g., from imperfect AO plus scintillation).

Multiple-Aperture Optical Receiver with Non-Coherent Combining andDemodulation

FIG. 4 is a schematic drawing of a receive-side architecture for aremote terminal, including a plurality of apertures 451 (e.g., eachcorresponding to a telescope) and configured to perform non-coherentaperture combining and demodulation (e.g., for direct detection ordifferentially coherent detection). A received downlink signal passesthrough the apertures 451, each of which is optically coupled to acorresponding tracking filter (452A-452K). Each filtered signal iscoupled to a fiber optic channel and routed to a corresponding opticalamplifier 453 for pre-amplification. Each pre-amplified signal is thende-multiplexed by a corresponding De-WDM 454 into several signalchannels that are detected by detectors 456A-456N. In other words, thecolors of each telescope's received signal are separated by thepre-amplified De-WDMs, and then each channel on each telescope isdetected. At that point, “like” channels can be combined bypost-detection combiners 457A-457N, for example, by summing therespective detector output voltages, or by digitizing and then digitallycombining them, either by soft decision or hard decision decoding. Thepost-detection combiners 457A-457N output corresponding combined signalsto receivers 450A-450N.

Multiple-Aperture Optical Receiver with Coherent Combining andDemodulation

FIG. 5 is a schematic drawing of a receive-side architecture includingapertures optically coupled to respective optical stretchers, andconfigured to perform coherent aperture combining and demodulation. Areceived downlink signal passes through the apertures 551, each of whichis optically coupled to a tracking filter 552 that filters a portion ofthe incoming downlink signal into a plurality of filtered signals. Eachfiltered signal is coupled to a corresponding fiber optic channel (“F”)for routing to a stretcher 558 or other delay element (e.g., a phasemodulator) that delays the signal in time or phase, depending in part onthe signal bandwidth and device geometry. For apertures mounted on asingle gimbal (such that the path delay between the apertures does notvary with pointing angle), the stretcher 558 can be viewed as aproviding variable phase delay that compensates for the relative phasebetween the apertures. For embodiments that might have the apertures onseparate gimbals, the stretcher also compensates for angle-dependentrelative time delay between the apertures.

A coherent beam combiner 557 coupled to the stretcher 558 combines thesignals into a combined beam. The combined beam is fiber opticallyrouted to a DeWDM 554 where it is de-multiplexed into colored signalsthat are fed to respective receivers 550A-550N; (i.e., a separatereceiver for each color). A feedback loop, comprising a sensor 559between the coherent beam combiner 557 and at least one stretcher 558monitors the relative delay between the signals from the variousapertures and adjust the corresponding stretcher(s) to compensate. Alocal oscillator (LO) bank 555 is operatively coupled to the receivers550A-550N and is configured to perform a coherent down-conversion forone or more of the colored de-multiplexed signals received at theplurality of receivers 550A-550N.

FIG. 6 is a schematic drawing of a receive-side architecture includingmultiple apertures optically coupled to respective optical amplifiersand optical stretchers, and configured to perform coherent aperturecombining and demodulation, according to an embodiment. The architectureof FIG. 6 functions as described with reference to FIG. 5 above, withthe exception that the pre-amplification (by optical amplifiers 653)occurs after each of the apertures 651 instead of after combining viathe coherent beam combiner 657. The architecture in FIG. 6 may be usedwhen the losses associated with the stretchers and the coherent combinerare not negligible.

Multiple-Aperture Optical Receiver with Channel-Wise Coherent ApertureCombining and Coherent Demodulation

FIG. 7 is a schematic drawing of a receive-side architecture includingmany apertures, and configured to perform channel-wise aperturecombining and coherent demodulation, according to an embodiment. Areceived downlink signal passes through the plurality of apertures 751each optically coupled to a tracking filter 752 that filters a portionof the incoming downlink signal into a plurality of filtered signals.Each filtered signal is coupled to a corresponding fiber optic channelfor routing to an optical amplifier 753 for pre-amplification. Eachpre-amplified signal is then de-multiplexed by a corresponding De-WDM754 into a plurality of signal channels that are each fiber-opticallyfed into respective stretchers 758 that apply phase adjustments to thesignals, and “like” channels are combined by a coherent beam combiner757 into a combined beam. In other words, wavelength de-multiplexing ofthe signal colors occurs near the apertures, a coherent optical combiner757 is assigned for each color, and a single fiber for each colordelivers its corresponding colored signal to a dedicated receiver 750A.750N. A feedback loop, comprising a sensor 759, between each coherentbeam combiner 757 and at least one corresponding stretcher 758 monitorsthe relative delay between the signals from the various apertures andadjust the corresponding stretcher(s) to compensate. The coherentcombiners 757 output corresponding combined signals to optical receivers750A-750N (“N” being the number of wavelengths). As in previousembodiments, a local oscillator (LO) bank 755 is operatively coupled tothe plurality of optical receivers 750A-750N and is configured toperform a coherent down-conversion for one or more of the colored,de-multiplexed, combined signals received at/detected by the pluralityof optical receivers 750A-750N.

Multiple-Aperture Optical Receiver with Intermediate FrequencyChannel-Wise Aperture Combining and Coherent Demodulation

FIG. 8 is a schematic drawing of a receive-side architecture including aplurality of apertures, and configured to perform intermediate frequency(IF) coherent channel-wise aperture combining and coherent demodulation,according to an embodiment. A received downlink signal passes throughthe plurality of apertures 851 each optically coupled to a trackingfilter 852 that filters a portion of the incoming downlink signal into aplurality of filtered signals. Each filtered signal is coupled to acorresponding fiber optic channel for routing to an optical amplifier853 for pre-amplification. Each pre-amplified signal is thende-multiplexed by a corresponding De-WDM 854 into a plurality of coloredsignal channels that are each down-converted to an electrical IF signalby a corresponding heterodyne (861A-861N) in conjunction with LO bank855. The IF signals are then coherently combined (by coherent combiners857) in the electrical domain and received at a corresponding IFreceiver for each arm (850A-850N), Because the down-conversion is to anelectrical IF, the subsequent combining and processing is non-optical.

Multiple-Aperture Optical Receiver with Digitally Coherent Channel-WiseAperture Combining and Coherent Demodulation

FIG. 9 is a schematic drawing of a receive-side architecture including aplurality of apertures, and configured to perform digitally coherentchannel-wise aperture combining and coherent demodulation, according toan embodiment. The architecture of FIG. 9 is similar to the architectureof FIG. 8, with the exception that each down-conversion (performed by LObank 955) is I-Q sampled (973A-973N) and then all of the I-Q samples,corresponding to the plurality of apertures 951, are coherently combineddigitally (at digital coherent combiners 957) before final demodulation.

Adaptive Optics to Compensate for Coupling Losses and Fading

As discussed above, atmospheric effects such as clouds, temperaturegradients and turbulence can have deleterious effects on free-spaceoptical transmissions, for example, such that light arriving at areceiver telescope of a remote terminal from an orbiting spacecraft isoptically aberrated or distorted. To compensate for atmospheric effects,adaptive optics technologies can be implemented at the receiver.Adaptive optics can include, for example, a feedback loop that includesa wavefront sensor and deformable mirror, spatial light modulator, orother device for changing the spatial phase profile of the receivedoptical signal. Because the remote terminal, during operation, slews atan angular velocity that exceeds that of typical astronomicalobservations (which slew at a much slower rate to compensate for theEarth's rotation), the bandwidths of the adaptive optic sensors andactuators should be large enough to compensate for the faster apparentchanges in the atmosphere due to the fast slew rate.

FIG. 10A is a block diagram showing components of an adaptive opticsassembly 1041. As shown in FIG. 10A, a light beam 11, relayed from atelescope and which has been aberrated due to atmospheric effects, isincident on a fast steering mirror 1071 that is steered and/or otherwisecontrolled, via an electrical connection, by a quad cell detector 1074so as to maximize the received signal. The incident light beam reflectedoff the fast steering mirror 1071 towards a beam splitter 1072 such thata first portion of the light beam is focused, via a lens onto the quadcell detector 1074, and a second portion of the light beam istransmitted to a deformable mirror 1075.

The deformable mirror 1075, under the control of a wavefront sensorfocal plane array 1077, modulates the spatial phase profile of theincident beam 11 to remove or reduce the aberrations. It reflects asubstantially or fully collimated beam 13 that is then routed to anotherbeam splitter 1076. The other beam splitter 1076 splits the collimatedbeam 13 such that a first portion of the collimated beam is focused, viaa lens L2, onto the wavefront sensor focal plane array 1077, and suchthat a second portion of the collimated beam is transmitted, focused viaa lens L3, and coupled into a single-mode fiber that routes the beam,via a modem pre-amplifier 1053, to a receiver modem 1078.

The deformable mirror 1075 is controlled, via an electrical connection,by the wavefront sensor focal plane array 1077 (e.g., a Shack-Hartmannwavefront sensor), and is configured to accomplish wavefrontreconstruction of a beam incident on the deformable minor 1075. Morespecifically, a lenslet array 1079 focuses different portions of theincident beam onto respective groups of sensor elements of the focalplane array 1077. A processor 1080 coupled to the focal plane array 1077estimates the local tilts of the incident wavefront from the positionsof the focused sports. These local tilts represent the aberration of thewavefront. The processor 1080 uses this estimate of the aberration tocontrol actuators of the deformable mirror 1075 so as adjust their axialpositioning to compensate for the detected aberration.

Other adaptive optics implementations are also possibly, including amuch simpler fiber nutator that gives lesser, but possibly adequate,performance. With a fiber nutator, the phase corrections for various“sub-apertures” are dithered at distinguishable frequencies. The powercoupled to the fiber can then be monitored at each of those frequenciesand used to optimize each of the phase corrections. This works well forreasonably small numbers of “sub-apertures” small (or, equivalently, ifD/r0 is not to large) and the phase correctors can dither at a frequencyhigher than the bandwidth of the corrections. It has the advantage ofnot requiring a high-speed camera as the power coupled to the fiber isused to monitor all signals.

FIG. 10B is a rendering of the wavefront sensor 1077 of FIG. 10A sensinga wavefront with no turbulence. Without turbulence, the beam incident onthe lenslet array 1079 is substantially or fully collimated. Under suchconditions, the lenslets 1079 subdivide and focus the incident lightbeam into a plurality of regularly arrayed spots that are centered ornearly centered on each corresponding group of sensor elements of thefocal plane array 1077.

By contrast, FIG. 10C shows the wavefront sensor of FIG. 10A sensing anaberrated wavefront. As shown, the incoming beam 11 is aberrated, forexample, due to atmospheric conditions such as clouds and/or airturbulence. Under such conditions, the lenslets 1079 subdivide and focusthe incident light beam into a plurality of spots that are scatteredacross the focal plane array 1077 in a non-uniform pattern. Theprocessor 1080 calculates the distance and direction of each spot fromits nominal position (shown in FIG. 10B) and uses them to estimate thephase aberration of the incident beam 11, as well as the correspondingactuator control corrections to be applied to the deformable mirror1075, as shown in the control loop of FIG. 10D.

The schematic drawings of FIG. 10E show the adaptive optics assembly ofFIG. 10A, as situated on the back end of a 10-100 cm (e.g., 40 cm)telescope suitable for use in a remote terminal. (The optical axis ofthe telescope is perpendicular to the plane of FIG. 10E.) As shown inthe left-hand schematic, depicting the wavefront sensor optical circuit,the incident light beam enters the center aperture “A” (out-of-plane ofthe drawing) and is optically routed to the fast steering minor 1071,the deformable mirror 1075, and the wavefront sensor 1077, in thesequence disclosed above. The left-hand schematic depicts the receivefiber path, comprising the fast steeling mirror 1071, the deformablemirror 1075, and the receiver lens and fiber F, in sequence.

FIGS. 11A-11C illustrate simulated effects of atmospheric turbulence oncoupling efficiency and fading. They also showed simulated performanceimprovements using adaptive optics like those shown in FIGS. 10A-10E.

The curve in FIG. 11A depicts the average coupling loss between lightentering a telescope and an optical fiber when only tilt tracking, viaan FSM, is used to compensate for atmospheric distortions. In this plot,D is the telescope's aperture diameter and r0 is the spatial coherencelength in the atmosphere, sometimes referred to as the Fried parameter.As the ratio D/r0 becomes large, the coupling loss becomes large. Forexample, for a 40-cm aperture, the stars labelled “Case 1A” and “Case2A” correspond to two different atmospheres, respectively: a “nominal”atmosphere in Which r0=10.1 cm, and a “stressing” atmosphere in Whichr0=7.8 cm (i.e., a reduced spatial coherence length). The curve showsthat the coupling loss to the fiber would be about 12-15 dB in thosecases. The stars labelled “Case 1B” and “Case 2B” represent what couldbe achieved with additional adaptive optics (including a deformablemirror and wavefront sensor) for the same nominal and stressingatmospheres, respectively. As can be seen in FIG. 11A, the couplingefficiency under the same respective conditions as Cases 1A and 2A isgreatly improved (by about 10 dB) with additional adaptive optics inCases 1B and 2B.

Atmospheric effects can also reduce the power of the transmitted lightbeam and/or cause “dropouts” during which transmitted data is notreceived at all. To compensate for such effects, forward errorcorrection (FEC) can be implemented at the optical receiver and/or theoptical transmitter as described above.

FIG. 11B shows a simulation of the time-variation of the coupling losses(normalized to the average coupling loss at 0 dB) for the nominal andstressing atmospheres of FIG. 11A. The gray curve is Case 1 (nominal)and the black curve is Case 2 (stressing). The bold horizontal lineshows the receiver threshold, assuming that the link has 3 dB of marginon the average power. When the gray and black curves drop below the boldhorizontal line, the received power is below the threshold power of thereceiver and bit errors or frame errors are likely to be present. Thesystem may correct for these with FEC techniques (see, e.g., thediscussion of box 167 of FIG. 1B above) and/or feedback protocols usedto retransmit frames that have errors.

FIG. 11C is a plot showing cumulative distribution functions for fadedepths for the nominal and stressing atmospheres of FIG. 11A. Thevertical dashed line at 3 dB shows the fades that go below the receiverthreshold for a system designed with 3 dB of margin on the averagepower. The curves show that under the stressing atmospheric conditions,the power drops below this threshold about 1% of the time, and under thenominal atmospheric conditions, the power drops below this thresholdabout 0.6% of the time. These percentages determine what FEC and/orfeedback protocols may be used to provide reliable data deliver over thechannel.

Conclusion

While various inventive embodiments have been described and illustratedherein, those of ordinary skill in the art will readily envision avariety of other means and/or structures for performing the functionand/or obtaining the results and/or one or more of the advantagesdescribed herein, and each of such variations and/or modifications isdeemed to be within the scope of the inventive embodiments describedherein. More generally, those skilled in the art will readily appreciatethat all parameters, dimensions, materials, and configurations describedherein are meant to be exemplary and that the actual parameters,dimensions, materials, and/or configurations will depend upon thespecific application or applications for which the inventive teachingsis/are used. Those skilled in the art will recognize, or be able toascertain using no in ore than routine experimentation, many equivalentsto the specific inventive embodiments described herein. It is,therefore, to be understood that the foregoing embodiments are presentedby way of example only and that, within the scope of the appended claimsand equivalents thereto, inventive embodiments may be practicedotherwise than as specifically described and claimed. Inventiveembodiments of the present disclosure are directed to each individualfeature, system, article, material, kit, and/or method described herein.In addition, any combination of two or more such features, systems,articles, materials, kits, and/or methods, if such features, systems,articles, materials, kits, and/or methods are not mutually inconsistent,is included within the inventive scope of the present disclosure.

The above-described embodiments can be implemented in any of numerousways. For example, embodiments of designing and making the technologydisclosed herein may be implemented using hardware, software or acombination thereof. When implemented in software, the software code canbe executed on any suitable processor or collection of processors,whether provided in a single computer or distributed among multiplecomputers.

Further, it should be appreciated that a computer may be embodied in anyof a number of forms, such as a rack-mounted computer, a desktopcomputer, a laptop computer, or a tablet computer. Additionally, acomputer may be embedded in a device not generally regarded as acomputer but with suitable processing capabilities, including a PersonalDigital Assistant (PDA), a smart phone or any other suitable portable orfixed electronic device.

Also, a computer may have one or more input and output devices. Thesedevices can be used, among other things, to present a user interface.Examples of output devices that can be used to provide a user interfaceinclude printers or display screens for visual presentation of outputand speakers or other sound generating devices for audible presentationof output. Examples of input devices that can be used for a userinterface include keyboards, and pointing devices, such as mice, touchpads, and digitizing tablets. As another example, a computer may receiveinput information through speech recognition or in other audible format.

Such computers may be interconnected by one or more networks in anysuitable form, including a local area network or a wide area network,such as an enterprise network, and intelligent network (IN) or theInternet. Such networks may be based on any suitable technology and mayoperate according to any suitable protocol and may include wirelessnetworks, wired networks or fiber optic networks.

The various methods or processes (e.g., of designing and making thetechnology disclosed above) outlined herein may be coded as softwarethat is executable on one or more processors that employ any one of avariety of operating systems or platforms. Additionally, such softwaremay be written using any of a number of suitable programming languagesand/or programming or scripting tools, and also may be compiled asexecutable machine language code or intermediate code that is executedon a framework or virtual machine.

In this respect, various inventive concepts may be embodied as acomputer readable storage medium (or multiple computer readable storagemedia) (e.g., a computer memory, one or more floppy discs, compactdiscs, optical discs, magnetic tapes, flash memories, circuitconfigurations in Field Programmable Gate Arrays or other semiconductordevices, or other non-transitory medium or tangible computer storagemedium) encoded with one or more programs that, when executed on one ormore computers or other processors, perform methods that implement thevarious embodiments of the invention discussed above. The computerreadable medium or media can be transportable, such that the program orprograms stored thereon can be loaded onto one or more differentcomputers or other processors to implement various aspects of thepresent invention as discussed above.

The terms “program” or “software” are used herein in a generic sense torefer to any type of computer code or set of computer-executableinstructions that can be employed to program a computer or otherprocessor to implement various aspects of embodiments as discussedabove. Additionally, it should be appreciated that according to oneaspect, one or more computer programs that when executed perform methodsof the present invention need not reside on a single computer orprocessor, but may be distributed in a modular fashion amongst a numberof different computers or processors to implement various aspects of thepresent invention.

Computer-executable instructions may be in many forms, such as programmodules, executed by one or more computers or other devices. Generally,program modules include routines, programs, objects, components, datastructures, etc. that perform particular tasks or implement particularabstract data types. Typically the functionality of the program modulesmay be combined or distributed as desired in various embodiments.

Also, data structures may be stored in computer-readable media in anysuitable form. For simplicity of illustration, data structures may beshown to have fields that are related through location in the datastructure. Such relationships may likewise be achieved by assigningstorage for the fields with locations in a computer-readable medium thatconvey relationship between the fields. However, any suitable mechanismmay be used to establish a relationship between information in fields ofa data structure, including through the use of pointers, tags or othermechanisms that establish relationship between data elements.

Also, various inventive concepts may be embodied as one or more methods,of which an example has been provided. The acts performed as part of themethod may be ordered in any suitable way. Accordingly, embodiments maybe constructed in which acts are performed in an order different thanillustrated, which may include performing some acts simultaneously, eventhough shown as sequential acts in illustrative embodiments.

All definitions, as defined and used herein, should be understood tocontrol over dictionary definitions, definitions in documentsincorporated by reference, and/or ordinary meanings of the definedterms.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Multiple elements listed with“and/or” should be construed in the same fashion, i.e., “one or more” ofthe elements so conjoined. Other elements may optionally be presentother than the elements specifically identified by the “and/or” clause,whether related or unrelated to those elements specifically identified.Thus, as a non-limiting example, a reference to “A and/or B”, when usedin conjunction with open-ended language such as “comprising” can refer,in one embodiment, to A only (optionally including elements other thanB); in another embodiment, to B only (optionally including elementsother than A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc.

As used herein in the specification and in the claims, “or” should beunderstood to have the same meaning as “and/or” as defined above. Forexample, when separating items in a list, “or” or “and/or” shall beinterpreted as being inclusive, i.e., the inclusion of at least one, butalso including more than one, of a number or list of elements, and,optionally, additional unlisted items. Only terms clearly indicated tothe contrary, such as “only one of” or “exactly one of,” or when used inthe claims, “consisting of,” will refer to the inclusion of exactly oneelement of a number or list of elements. In general, the term “or” asused herein shall only be interpreted as indicating exclusivealternatives (i.e. “one or the other but not both”) when preceded byterms of exclusivity, such as “either,” “one of,” “only one of,” or“exactly one of” “Consisting essentially of,” when used in the claims,shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” “composed of,” and the like are tobe understood to be open-ended, i.e., to mean including but not limitedto. Only the transitional phrases “consisting of” and “consistingessentially of” shall be closed or semi-closed transitional phrases,respectively, as set forth in the United States Patent Office Manual ofPatent Examining Procedures, Section 2111.03.

The invention claimed is:
 1. An apparatus for receiving a modulatedoptical signal transmitted from a spacecraft in low-Earth orbit (LEO) ormedium-Earth orbit (MEO) via a free-space optical communicationschannel, the apparatus comprising: a controller to determine when thespacecraft will reach a predetermined angle above a horizon; an uplinktransmitter, operably coupled to the controller, to tripper transmissionof the modulated optical signal by the spacecraft when the spacecraftreaches the predetermined angle above the horizon so as to reducecoupling loss of the modulated optical signal due to atmosphericturbulence; at least one telescope to receive the modulated opticalsignal from the spacecraft via the free-space optical communicationschannel, the modulated optical signal being modulated at a rate of atleast 40 Gigabits per second (Gbps); a single-mode waveguide, in opticalcommunication with the at least one telescope, to guide the modulatedoptical signal; an optical amplifier, in optical communication with thesingle-mode waveguide, to amplify the modulated optical signal; and areceiver, in optical communication with the optical amplifier, todemodulate the modulated optical signal at a rate of at least 40 Gbps.2. The apparatus of claim 1, wherein the at least one telescope definesat least one aperture having a diameter from 10 centimeters to 100centimeters.
 3. The apparatus of claim 1, wherein the at least onetelescope comprises a plurality of apertures, each of which receives acorresponding portion of the modulated optical signal.
 4. The apparatusof claim 1, wherein the modulated optical signal comprises awavelength-division multiplexed (WDM) optical signal and the receivercomprises: a wavelength-division demultiplexer, in optical communicationwith the optical amplifier, to separate the WDM optical signal into aplurality of optical signals; and a plurality of detectors, in opticalcommunication with the wavelength-division demultiplexer, to demodulatethe plurality of optical signals.
 5. The apparatus of claim 1, whereinthe uplink transmitter is further configured to transmit identificationand validation information to the spacecraft.
 6. The apparatus of claim1, wherein the predetermined angle is at least 20 degrees above thehorizon.
 7. The apparatus of claim 5, wherein: the controller isoperably coupled to the receiver and the uplink transmitter and isconfigured to sense at least one error associated with the modulatedoptical signal, and the uplink transmitter is configured to triggeranother transmission, by the spacecraft, of at least a portion of themodulated optical signal in response to the at least one error.
 8. Theapparatus of claim 1, further comprising: an adaptive optical element,in optical communication with the at least one telescope and thesingle-mode waveguide, to modulate a spatial phase profile of themodulated optical signal so as to compensate for atmospheric effects inthe free-space optical communications channel.
 9. The apparatus of claim1, further comprising: a coherent source, in optical communication withthe receiver, to coherently demodulate the modulated optical signal. 10.A method of free-space optical communication with a spacecraft inlow-Earth orbit (LEO) or medium-Earth orbit (MEO), the methodcomprising: determining when the spacecraft will reach a predeterminedangle above a horizon during an orbit; triggering transmission of amodulated optical signal by the spacecraft when the spacecraft reachesthe predetermined angle above the horizon so as to reduce coupling lossof the modulated optical signal due to atmospheric turbulence; receivingthe modulated optical signal transmitted from the spacecraft via afree-space optical communications channel, the modulated optical signalbeing modulated at a rate of at least about 40 Gigabits per second(Gbps): amplifying the modulated optical signal; and demodulating themodulated optical signal at a rate of at least 40 Gbps.
 11. The methodof claim 10, wherein receiving the modulated optical signal comprisescollecting light via at least one aperture having a diameter from 10centimeters to 100 centimeters.
 12. The method of claim 10, whereinreceiving the modulated optical signal comprises coupling light from atelescope to a single-mode waveguide.
 13. The method of claim 10,wherein receiving the modulated optical signal comprises modulating aspatial phase profile of the modulated optical signal so as tocompensate for atmospheric effects in the free-space opticalcommunications channel.
 14. The method of claim 10, wherein themodulated optical signal comprises a wavelength-division multiplexed(WDM) optical signal, and wherein receiving the modulated optical signalcomprises separating the WDM optical signal into a plurality of opticalsignals.
 15. The method of claim 14, wherein demodulating the modulatedoptical signal comprises demodulating each optical signal in theplurality of optical signals.
 16. The method of claim 15, whereindemodulating the plurality of optical signals comprises coherentlydemodulating at least one optical signal.
 17. The method of claim 10,further comprising: transmitting identification and validationinformation to the spacecraft prior to receiving the modulated opticalsignal.
 18. The method of claim 10, wherein triggering transmission ofthe modulated optical signal by the spacecraft occurs when thespacecraft is at an elevation of at least 20 degrees above the horizon.19. The method of claim 10, further comprising: sensing at least oneerror associated with the modulated optical signal; and triggeringanother transmission, by the spacecraft, of at least a portion of themodulated optical signal in response to the at least one error.
 20. Themethod of claim 10, further comprising: ending transmission of themodulated optical signal when the spacecraft falls below a specifiedelevation angle.